Time domain reflectometry (TDR) and time domain transmissiometry (TDT) are commonly used techniques to characterize transmission lines. TDR is used to locate discontinuities in the transmission lines. The discontinuities cause reflections and appear as peaks on the TDR waveform. The shape of the each peak represents characteristics of the discontinuity. For example, a frequency response of a standalone discontinuity can be extracted from the TDR waveform by a technique called gating, which masks the TDR waveform, leaving the peaks as is and filling other portions of the TDR waveform with zeros. The masked TDR waveform is transformed into the frequency domain by Fourier transform to reproduce the frequency response of the standalone discontinuity. TDT is used to measure an electrical delay of the transmission line. The distance from a reference plane to a first peak in the TDT waveform corresponds to the electrical delay of the transmission line.
If there was no loss (attenuation) and/or dispersion in the transmission line, the peaks would be sharp. However, real transmission lines have loss and dispersion that flatten the peaks, and make it difficult to determine the locations and the magnitudes of the peaks in the TDR/TDT waveforms. Gating does not work effectively if the peaks of the TDR waveform are flattened.
There have been attempts to prevent the occurrence of flattened peaks in TDR/TDT waveforms due to transmission line loss and/or dispersion. Typical approaches have not been very successful, especially with respect to loss compensation, and generally result in unrealistic TDR/TDT waveforms. For instance, typical approaches often assume that the reflection coefficient of each discontinuity is constant with respect to frequency. However, lumped elements such as capacitors and inductors have frequency dependent characteristics and produce peaks accompanied by long tails in the TDR waveform. The assumption of a constant reflection coefficient is not valid for these lumped elements. Other typical approaches have overcompensation problems, which cause the TDR/TDT waveform to diverge (increase or decrease) as the distance from the reference plane increases, even if no discontinuity or variation in impedance of the transmission line occurs.